For some years, the demand for “clean” vehicles has tended to increase as a result of the needs firstly to reduce fuel consumption and secondly to limit pollution.
In general, hybrid systems and micro-hybrid systems are being developed for the purpose of fulfilling the aforementioned needs.
Micro-hybrid systems are known, for example, with recuperative braking, wherein an alternator is used to collect mechanical torque, thus giving rise to braking of the vehicle. The alternator converts this collected torque into electrical energy, in order to charge an energy storage device in the form for example of a pack of super-capacitors or a battery. This energy which is recuperated is then returned to the various items of electrical and electronic equipment which the motor vehicle includes. In so-called “14+X” micro-hybrid systems with floating direct voltage, this energy can also be used to start the thermal engine, or to assist the torque of this thermal engine.
However, the integration of this type of micro-hybrid system in the engine compartment of a modern motor vehicle may cause problems. In fact, a micro-hybrid system consists of elements which must be interconnected with one another, some of these elements being able to be relatively bulky. Since the engine compartment of a motor vehicle has a relatively limited amount of space, it is becoming increasingly difficult for car manufacturers to incorporate new systems into them. This results is a certain number of technical choices, such as the fact of moving the energy storage device away from the other elements of the micro-hybrid system, for example by installing this device in the boot. Thus, the lengths of branching cables which form the power bus can be substantial, and introduce parasitic inductances of a type which is to the detriment of the micro-hybrid system in switched dynamic running conditions.
The power bus which is placed between the AC-CD converter of the micro-hybrid system and the energy storage device poses a particular problem. In fact, substantial pulse currents can be conveyed via this power bus, between the AC-DC converter and the energy storage device. For example, substantial pulse currents intervene during the functioning in starter mode of the rotary electric machine. The parasitic inductance of this power bus can firstly affect the energy performance at certain frequencies, and can also give rise to excess resonance voltages. In addition, the parasitic inductance can affect adversely the electromagnetic compatibility.
The excess resonance voltages can give rise to uncontrolled avalanche phenomena in MOSFET power transistors of the AC-DC converter, these avalanche phenomena being able to detract from the functioning of these transistors, or damage them. The reliability of the micro-hybrid system can therefore be reduced greatly by these avalanche phenomena.
In the state of the art, it is known to use as a power bus a cable consisting of two juxtaposed insulated cylindrical conductors. This type of cable permits reduction of the parasitic inductance in comparison with other wiring solutions, such as wiring which involves a single conductor which forms a positive core, and requires a return via the bodywork of the motor vehicle which forms a negative core, this return acting as an earth. For example, for a cable length of 3 m, inductance of approximately 3 μH is obtained.
The aforementioned standard cable can be used in micro-hybrid systems for currents which can be as much as 600 A, particularly in the starter mode of the thermal engine, as a result of the presence in the AC-DC converter of a capacitor with a few tens of μF for example 60 μF, constituting a passive filter which limits the excess voltages.
For micro-hybrid systems with currents above 600 A, with this standard cable which has a parasitic inductance of approximately 3 μH for a length of 3 m, a capacitor with a far greater capacity is necessary. For example, in a known micro-hybrid system which functions with currents of approximately 1100 A, a capacitor of approximately 2000 μF may be necessary at the level of the AC-DC converter. Since this capacitor must preferably be integrated in the AC-DC converter, there is an integration constraint which is difficult to fulfil because of the size of the capacitor.